A massive swell of warm water is buffeting South
America. Is it the first sign of a new El Niño -- or just
another "Kelvin wave?"

March
5, 2002: Somewhere on a beach in Ecuador or Peru, someone
is out for stroll. A warm breeze is blowing, water laps at their
feet. It's paradise.

But something is amiss. The air is a bit warmer than usual.
So is the water. And, for weeks now, tides have been cresting
a few inches higher. Maybe only frequent beach-walkers would
notice the difference, but the changes are real.

These countries have just been hit by a gentle yet massive
swell of warm water -- a so-called "Kelvin wave."

"Kelvin waves are warm bumps in the Pacific Ocean,"
says JPL oceanographer Bill Patzert. They form around Indonesia
and travel east toward the Americas. "A typical Kelvin wave
is 5 or 10 cm high, hundreds of kilometers wide, and a few degrees
warmer than surrounding waters."

Usually not much happens when a Kelvin wave arrives --
beach combers experience a bit of extra rain, perhaps, and slightly
warmer surf. Nevertheless, scientists pay careful attention to
them because these gentle waves occasionally herald something
far more powerful: the next El Niño.

El Niños and Kelvin waves are both triggered by winds
-- or a lack thereof -- in the Pacific Ocean. Patzert explains:
Pacific trade winds blow from east (the Americas) to west (Indonesia).
The persistent breeze pushes Sun-warmed surface waters westward
and, as a result, the sea level near Indonesia is normally 45
cm higher than it is near Ecuador.

"We call that part of the ocean near Indonesia the warm pool -- it's
the biggest area of warm water on our planet," says David
Adamec, a climate researcher at NASA's Goddard Space Flight Center.
The pool heats the atmosphere above the western Pacific where
convection cells spawn thunderstorms, lightning, and plenty of
rain.

Above: A Kelvin wave crosses the Pacific, Dec. 8, 2001
- Mar. 3, 2002. This plot shows a 500m-deep slice of the Pacific
Ocean along the equator. The warm
pool is on the left, the Americas are on the right. False
colors denote temperature anomalies (measured by Pacific ocean
buoys) in degrees Celsius. "The warmest part of a Kelvin
wave is usually 100 to 150 meters below the ocean's surface,"
notes Adamec. [more]

Sometimes the trade winds falter for a few days or weeks --
researchers aren't sure why -- and warm water slides back across
the Pacific toward the Americas. "That's a Kelvin wave,"
says Patzert. "We see at least one of them every (northern)
winter."

El Niño begins when trade winds falter -- not just
briefly -- but for many months. Then, says Patzert, strong Kelvin
waves cross the Pacific "like a conveyor belt," depositing
warm water near South America where the ocean is normally cold.
This new "warm pool" alters weather all over our planet.
Rains that would normally soak the western Pacific shift toward
the Americas, while places like Indonesia and India become dry.

Some people welcome the change. The last El Niño in
1997 suppressed Atlantic hurricanes, lowered winter heating bills
in New England, and drew surfers to suddenly warm beaches in
California. Sled dogs in the Sierra Nevada enjoyed extra sledding
when it snowed in June!

Others dread it. Coastal villagers in South America fled torrential
rains in 1997 that washed away their homes. Many were ready to
leave anyway to escape record swarms of mosquitoes that bred
in standing pools. Meanwhile, normally wet places like Indonesia
were hit by terrible droughts and wildfires.

Below: Flash floods can be a side-effect of strong El
Niños. Credit: NOAA.

"El
Niño can do a lot of damage," acknowledges Patzert,
"but it's not all bad." For example, farmers can profit
from changing weather patterns by cultivating "El Niño
crops." Rice and beans, for instance, could be planted in
areas normally too dry to support them. But such is only possible
if growers know when El Niño is coming.

Scientists have developed computer models that do a good job
forecasting El Niño's effects -- after El Niño
is underway. "The hard part," says Adamec, "is
predicting when El Niño will begin."

Indeed, life would be easier if El Niño came and went
on a regular schedule, but the interval between El Niños
varies from 3 to 7 years -- another mystery for researchers.
Because the last one began in 1997, the next could begin as early
as this year or as late as 2004. No one knows.

After the El Niño of 1982-83 took forecasters by surprise,
the United States, Japan, and France strung 70 buoys across the
equatorial Pacific. Called the "TAO array," these buoys
monitor water temperature to a depth of 500 meters, as well as
winds, air temperature, and relative humidity. They are designed
to be an early warning system for El Niño.

Below: One of 70 buoys in the Tropical Atmospheric Ocean
(TAO) array. [more]

El
Niño warnings also come from the US-French TOPEX/Poseidon
satellite. It can measure the height of the ocean's surface by
"pinging it" with an onboard radar. Warm water expands,
so Kelvin waves appear as traveling bumps in the satellite's
sea surface elevation maps.

Both TAO and TOPEX/Poseidon have tracked the latest Kelvin
wave since it formed near Indonesia in Dec. 2001. "The wave
crossed the Pacific in January and reached South America in February,"
says Patzert. Not all Kelvin waves manage to cross the vast Pacific
-- but this one did. It reminds Patzert of another notable Kelvin
wave: "It looks a lot like one that crossed the Pacific
in early 1997 -- just before the last El Niño."

Does this mean another El Niño is near? "Not necessarily,"
he says. "Kelvin waves appear in the Pacific every winter;
they don't all mean El Niño is coming."

Adamec agrees: "It's too soon to say. The real test will
be what happens this spring." If the "conveyor belt"
starts turning as it did in the (northern) spring of 1997, then
we will know El Niño is back.

TOPEX/Poseidon
-- A partnership between the U.S. and France to monitor global
ocean circulation, discover the tie between the oceans and atmosphere,
and improve global climate predictions. Every 10 days, the TOPEX/Poseidon
satellite measures global sea level with unparalleled accuracy.

Eye on the Ocean -- (EarthObservtory) This article explains
how El Niño is formed and influenced by a constant exchange
of energy between the Pacific Ocean and our planet's atmosphere.